Switching regulators and linear regulators are well known types of voltage regulators for converting an unregulated voltage, such as a battery voltage, to a regulated DC voltage of a desired value. Some applications of voltage regulators include low noise DC-to-DC converter circuits for use in cell phones, PDA's (personal digital assistants), VCO (voltage controlled oscillator) and PLL (phase locked loop) power supplies, and smart card readers. One type of switching regulator is a pulse width modulation (PWM) regulator that turns a switching transistor on and off at a certain frequency. In a conventional buck regulator topology, the power supply voltage is intermittently coupled to an inductor, and the inductor conducts a triangular current waveform to recharge an output filter capacitor. The charged filter capacitor provides a relatively constant voltage to the load. A feedback signal, which is typically the output voltage, determines when to shut off the switching transistor during each switching cycle. The switch on-time percentage is called the duty cycle, and this duty cycle is regulated so as to provide a substantially constant voltage at the output despite load current changes. There are many types of switching regulators.
A linear regulator, also referred to as a low dropout (LDO) regulator, controls the conductance of a transistor in series between the unregulated power supply and the output terminal of the regulator. The conductance of the transistor is controlled based upon the feedback voltage to keep the output voltage at the desired level.
Switching regulators are generally considered to be more efficient than linear regulators since the switching transistor is either on or off. When a transistor is fully on, such as in saturation or near the edge of saturation, the transistor is a highly efficient switch, and there is a minimum of wasted power through the switch. However, due to the pulsing of the current through the switch, a relatively large size filter circuit, consisting of an inductor and a capacitor, is needed so as to provide a low-ripple regulated voltage at the output. The inductor must be sized to not saturate at the highest rated load current for the switching regulator under worst-case conditions. The size of the capacitor is based upon the frequency of the switching regulator and the allowable ripple. Accordingly, it is difficult to provide a very small switching regulator, including the filter circuitry, in a very small size while supplying a low-ripple regulated voltage.
A linear regulator, on the other hand, provides a very smooth output since the series transistor is always conducting. However, due to the large voltage differential across the transistor, power is wasted through the transistor, and substantial heat may be generated.
It is known to use a linear regulator at the output of a switching regulator to further smooth the output of the switching regulator for applications which require extremely steady regulated voltages. However, the resulting power supply is still relatively large due to the switching regulator inductor being sized so as not to saturate at the maximum load current under worst-case conditions. The size of the inductor and capacitor dominate the overall size of the regulator.
An additional issue that effects both linear and switching voltage regulators is the compensation needed on the feedback circuitry. The compensation is required to maintain stability in the regulation circuit, but it also limits the transient performance of those designs. Typical transient response for existing voltage regulator designs may be in the range of 10 to 100 μSec.
What is needed is a smaller size voltage regulator that supplies a very low amplitude ripple regulated output voltage with die size efficiency and shorter transient response times. In view of the ever-increasing commercial competitive pressures, along with growing consumer expectations and the diminishing opportunities for meaningful product differentiation in the marketplace, it is critical that answers be found for these problems. Additionally, the need to save costs, improve efficiencies and performance, and meet competitive pressures, adds an even greater urgency to the critical necessity for finding answers to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.